LaVO
4
, LaPO
4
and LaBO
3
: synthesis, Rietveld
refinement, and comparison of optical properties
G. ÇELIK GÜL*, F. KURTULUŞDepartment of Chemistry, Balik esir University, 10145, Balik esir, Turk ey
Rare earth metals luminescent materials have been practically applied in almost all devices from cathode ray tubes to coating materials. Lanthanum containing samples of orthovanadate, orthophosphate and orthoborate (LaVO4, LaPO4 and
LaBO3) compounds have been successfully prepared by microwave -assisted solid state synthesis (MASSS) method.
Characterizations were done by powder X-ray diffraction (XRD), Rietveld refinement method, fourier transform infrared (FTIR) spectroscopy, photoluminescence spectroscopy (PL) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS). The Rietveld refinement analyses by using XRD data reveal that LaVO4 and LaPO4 powders are
crystallized in monoclinic system; LaBO3 is in orthorhombic. The morphological results confirm the particle sizes are in the
range micron to nano-scale. While LaPO4 and LaBO3 powders exhibit green phosphor, LaVO4 seems yellow under visible
range.
(Received February 20, 2018; accepted February 12, 2019)
Keywords: Rare earth metals, Borates, Phosphates, Vanadates, Rietveld method, Optical property
1. Introduction
Luminescent materials especially containing rare earth metals have remarkable applicable in almost all devices such as X-ray detectors, lamps, cathode tubes [1], biomedical sensors and markers [2,3]. The complex relationships between structure, properties and stoichiometry must been well-understood to produce oxides [4]. Having the general formula RAO4 (R=rare earth metal, A=V, P, Ta, W etc.) rare earth compounds have been intensively emphasized for their physical and chemical properties [5]. This typical formula provides flexibility for cation substitution on both R and B sites [6]. The coordination number of R varies from structure to structure and there are wide variations in R‒O distances within a given polyhedron [7]. There are different kind of phosphor materials based on lanthanides, such as oxides, silicates, aluminates, vanadates, phosphates and borates [8]. The reason of intensive study about lanthanid e cations is exhibition photo luminescent properties due to a wide range of emission colors which are based on 4f‒4f or 5d‒ 4f transitions [1]. A group of them is rare earth orthovanadates and orthophosphates which have been revealed born of top level to apply cathode luminescent, low temperature transformations and Jahn-Teller distortion except a few of them . Besides, these compounds create wide practical field such as laser hosts, sensors and catalyst due to multiple transition of rare earths and electronic structure [9]. Lanthanide orthovanadates are major compounds in this system and exhibit both physical, magnetic and crystallographic properties [10]. Lanthanide
orthovanadates have two crystalline form: monoclinic monazite type and tetragonal zircon typ e [25,26]. In general, the larger lanthanide ion prefers monazite type owing to its higher oxygen coordination number compared to the zircon type [10]. Lanthanum is the first element of lanthanides with the largest atomic radii naturally should choose monazite type, but as an exception lanthanum prefers more stable zircon type [11]. Tetragonal lanthanum orthovanadate was first synthesized by Ropp and Carroll [12] as a low-crystalline form via conventional route. The corrections about some contradictory reports [13] confirmed that to obtain high crystalline form the soft chemical methods become compulsory, and are used in many researches [14]. In terms of features, lanthanum vanadates and phosphates are promising candidates as phosphor material according to Yan’s research [15]. These functional groups have ease charge transfer which make them as potential pigments and photocatalytic material [16]. Lanthanide orthovanadates are candidates on solar energy harvesting due to red shift in the excitation spectra. Low stability of orthovanadates and orthophosphates restrict their application in many fields, although all advantages [17]. Hence, lanthanum phosphates and lanthanum borates have been needed to investigate and compare to orthovanadates. The former lanth anum phosphates have the same crystal structure as lanthanum orthovanadates and exhibit dominant protonic conduction and high chemical stability [18]. Further, lanthanum phosphates have been chosen due to their potential applications on fluorescent lamps and color TV monitors [19]. The latter lanthanum borates are crystallized in
orthorhombic aragonite-type structure with space group Pnam, isomorphs with CaCO3, and display high temperature protonic conduction, excellent thermal and chemical stability, and high vacuum ultraviolet transparency [20]. On the other hand, RBO3 (R=rare earth metal) type anhydrous orthoborate compounds have been attracted a great interest for the preparing due to structural complexity because boron atoms to form all kinds of borates planar BO3, nonplanar BO3 and tetrahedral BO4 groups [21]. Many metal borates display important practices in nonlinear optical and laser applications . They have also significant magnetic, catalytic and phosphorescent properties [22]. Due to the mentioned applications, the metal borates find many technological practices in the industrial area.
The leadable high-temperature solid state reaction which alternates the structure is a common method to synthesis rare earth phosphor materials . For the rest, hydrothermal method and chemie douce have been used as wet methods [23]. The related methods have some disadvantages: high reaction temperature, limited homogeneity, complexity and high cost [24]. Therefore, improving a simple and low cost method is an essential necessity. Microwave assisted‒solid state synthesis (MASSS) can be an alternative method to obtain these types of compounds. MASSS shorten high temperature heating process, improve yield, and accelerate reaction time and most importantly reducing costs.
From the literature to the best of our knowledge, it is not possible to find any comparatively publications about microwave‒assisted synthesis, structural and luminescent properties of lanthanum vanadates, phosphates and borates. In this paper, we present the MASS synthesis of LaVO4, LaPO4 and LaBO3 compounds, comparison of characteristic properties such as: structural, morphological, thermal and luminescent. XRD, FTIR, SEM/EDS and PL of the all compounds have been used as characterization techniques to complete the comparison.
2. Experimental section
All chemical lanthanum (III) oxide, vanadium (V) oxide, phosphorus (V) oxide, and boron trioxide have been used analytical grade, and provided by Merck. LaVO4, LaPO4 and LaBO3 compounds were fabricated using a domestic microwave oven as described conditions: 850 W, 20 min. and 2.45 GHz. In terms of being an example, lanthanum orthovanadate has been synthesized as follows: lanthanum (III) oxide and vanadium (V) oxide in 1:1 molar ratio were weighted and homogenized in an agate mortar. Then, the mixture was laid along porcelain crucible, and treated microwave irradiation. After cooling down to room temperature, the mixture was regrounded, and crystallized at 450 ºC for 2 h to obtain best shaped crystals. The same procedure was applied to obtain the
other two compounds. The structural characterizations of LaVO4, LaPO4 and LaBO3 compounds were carried out powder X-ray diffraction (XRD) measurements using Panalytical X’Pert Pro Diffractometer and Cu Kα radiation (λ=1.54056 Ǻ, 40 mA, 50 kV) with a scan rate of 1º/min with step size 0.02º. The Rietveld analyses of the samples were done by using the High Score Plus (HS+) Program (License number: 92000029). Fourier transform infrared spectroscopy (FTIR) was achieved on a Perkin Elmer Spectrum 100 FTIR Spectrometer from 4000 to 650 cm-1. The morphological properties and chemical compositions of the samples were determined by means of SEM JEOL 6390-LV scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS). The luminescence properties were measured by Andor Solis Sr 500i spectrophotometer (VUV-PL) at grating 1200 and 100 μm slit conditions. Siemens V12 domestic microwave oven and Protherm conventional furnace were used wave and h eat treatment, respectively.
3. Results and discussion
The structural properties of LaVO4, LaPO4 and LaBO3 compounds have been studied using XRD spectra which are showed in Fig. 1. All diffractograms show only characteristic diffractions of LaVO4, LaPO4 and LaBO3 compounds and these peaks collaborate with ICSD card numbers 008-8294, 007-9747 and 003-5535, respectively. Other peaks belonging to lanthanum (III) oxide, vanadium (V) oxide, phosphorus (V) oxide, and boron trioxide are not observed in the patterns. All the peaks have been marked as related miller planes in Fig. 1. The observed and calculated “d” values of the compounds are given in Table 1, and the unit cell parameters of the compounds have been calculated by using these values via Rietveld Refinement Program (Table 2). The calculated unit cell parameters nearly agree with the card data.
Fig. 1. Powder X-ray diffraction pattern of LaVO4,
Table 1. The observed and calculated XRD data of LaVO4, LaPO4 and LaBO3 compounds
LaVO4 LaPO4 LaBO3
dobs. (Å) dcalc. (Å) h k l dobs. (Å) dcalc. (Å) h k l dobs. (Å) dcalc. (Å) h k l 5.4484 5.4443 1 0 0 5.2217 5.2245 1 0 0 4.3637 4.3359 0 1 1 4.8424 4.8431 1 1 -1 4.8437 4.8442 0 1 1 3.5042 3.4900 1 1 1 4.3597 4.3590 1 1 0 4.7140 4.7159 1 1 -1 3.3906 3.3757 1 0 2 4.1887 4.1897 1 0 -2 4.2005 4.2013 1 1 0 2.9400 2.9408 2 0 0 3.6345 3.6375 0 2 0 4.1347 4.1360 1 0 -2 2.5650 2.5486 0 2 0 3.6293 3.6307 1 1 -2 3.5693 3.5697 1 1 -2 2.4383 2.4337 2 1 1 3.4005 3.4010 0 0 2 3.5355 3.5340 0 2 0 2.4351 2.4148 1 0 3 3.2055 3.2077 0 2 1 3.3243 3.3260 0 0 2 2.3990 2.3940 0 1 3 3.0801 3.0810 0 1 2 3.1647 3.1655 2 0 -2 2.1819 2.1677 0 2 2 2.9864 2.9852 2 1 -1 3.1216 3.1209 0 2 -1 2.0456 2.0339 1 2 2 2.9642 2.9639 2 1 -2 3.0866 3.0858 1 2 -1 1.9567 1.9450 1 0 4 2.7241 2.7224 2 0 0 3.0084 3.0094 0 1 2 1.9328 1.9259 2 2 0 2.5509 2.5498 2 1 0 2.8886 2.8890 2 1 -2 1.8754 1.8754 2 2 1 2.5099 2.5104 2 1 -3 2.8789 2.8796 2 1 -1 1.7879 1.7863 3 1 1 2.2701 2.2718 1 3 -1 2.6109 2.6122 2 0 0 1.7723 1.7704 3 0 2 2.2352 2.2340 3 0 -2 2.4672 2.4675 2 1 -3 1.6108 1.6025 0 2 4 2.2235 2.2234 1 1 2 2.4492 2.4502 2 1 0 1.5557 1.5461 1 2 4 2.1863 2.1875 1 2 -3 2.2088 2.2080 1 3 -1 1.4581 1.4540 4 0 0 2.0125 2.0132 2 1 -4 2.1589 2.1596 3 0 -2 1.4546 1.4482 3 2 2 1.9717 1.9727 1 3 1 2.1525 2.1535 1 1 2 1.3038 1.2969 2 3 3 1.9480 1.9487 2 3 -1 2.1454 2.1455 1 2 -3 1.2733 1.2735 4 2 0 1.9235 1.9242 0 2 3 1.9843 1.9847 2 1 -4 1.2585 1.2586 4 2 1 1.8404 1.8402 2 2 1 1.9117 1.9115 1 3 1 1.2456 1.2447 2 0 6 1.7956 1.7966 2 3 3 1.8876 1.8873 2 3 -1 1.2298 1.2248 3 1 5 1.6962 1.6957 2 0 2 1.8778 1.8782 0 2 -3 1.1674 1.1692 2 4 0 1.6232 1.6241 3 2 0 1.8173 1.8118 3 2 -3 1.1249 1.1248 2 4 2
Table 2. Crystal system and unit cell parameters of LaVO4, LaPO4 and LaBO3 compounds calculated by
Rietveld refinement method using X-ray powder diffraction data
Compound Crystal system
Unit cell parameters a (Å) b (Å) c (Å) LaVO4 monoclinic 6.7161 7.2759 8.3003 LaPO4 monoclinic 6.5047 7.0678 8.2822 LaBO3 orthorhombic 5.8808 5.0965 8.2428
The FTIR spectrums of LaVO4, LaPO4 and LaBO3 compounds are shown in Fig. 2. The vibrations of LaVO4 in the range of 631-970 cm-1 are correspond to V‒O, V‒ O‒V and V=O subgroups of vanadate structure [25]. In the LaPO4 spectrum, the peaks at 690-700 cm-1, 1000-1050 cm-1, 1100-1138 cm-1 and 1180-1300 cm-1 are related to ʋs(PO4), ʋ3(PO4), ʋs(OPO) and ʋas(OPO) vibrations, respectively. In the third spectrum the peaks at 600-680 cm-1, 700-900 cm-1, 900-1000 cm-1 and 1000-1300 cm-1 are belong to in order δdi(BO3), δdd(BO3), ʋ1(BO3)/ʋs(BO3) and ʋas(BO3) of LaBO3 compound [26].
Fig. 2. The FTIR spectrums of LaVO4, LaPO4 and
In Fig. 3 SEM micrographs of LaVO4, LaPO4 and LaBO3 compounds are demonstrated. Fig. 4 is EDS results of the related compounds. Although homogeneous views of all samples are nearly similar, particle sizes are
changing nm to μm scale. While the particle size of lanthanum vanadate is about in 1-10 μm, dimensions of LaPO4 and LaBO3 powders are in nm scale.
Fig. 3. Scanning electron micrograph images of LaVO4, LaPO4 and LaBO3 compounds
In Fig. 5, PL emissions of LaVO4, LaPO4 and LaBO3 compounds are exhibited via black, red and green lines, respectively. In the black line, there are two transitions at 550-650 nm and 650-700 nm which are attributed to 5D
0→7F1 and 5D0→7F2 transitions, respectively [27]. The broad red line is related to several d-f transitions of the lanthanum phosphate [28]. The green line has two bands; first is broad band which display transitions of excited 5d states to the 2F5/2 ground state, and last is small band related to 5D0→7F4 transition [27,28].
Fig. 5. Photoluminescent spectrum of LaVO4, LaPO4
and LaBO3 compounds
4. Conclusion
The green or yellow-emitting phosphors LaVO4, LaPO4 and LaBO3 have been synthesized for the first time microwave-assisted solid state method by using oxides of all elements. The observed XRD data as “d” values have been compared to calculated “d” values by Rietveld refinement method. Also, unit cell parameters of LaVO4, LaPO4 and LaBO3 compounds have been calculated and crystal systems have been determined Rietveld refinement method. The vibrations in FTIR spectrums confirm the functional groups in the related compounds. The morphological images exhibit that the approximate particle size of lanthanum vanadate is 1-10 μm, meantime dimensions of LaPO4 and LaBO3 powders are in 1-10 nm. The luminescent emissions of LaVO4, LaPO4 and LaBO3 powders are yellow, blue/green and green colors.
Acknowledgement
This work has been financially supported by Balıkesir University with research project foundation and Scientific and Technological Research Council of Turkey. We want to thank to Assist. Prof. Dr. Sevim Şenol and Dr. M. Burak Çoban for scientific support.
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